There's a new website called Freerice.com. It gives you multiple choice vocabulary questions, and for every one you answer correctly, the site donates 20 grains of rice to the United Nations World Food Program. Advertising income pays for the donated rice.
According to Foreign Policy magazine, the site averages close to 200,000 hits daily, and since the site started, 6.9 billion grains have been donated.
Check it out-- the first couple questions are easy (for readers of my blog, at least), but around the fourth or fifth they starting getting a little obscure. A tip: if you have no idea what the word is, look for familiar root words or suffixes. If that doesn't work, go for the strangest or most specific answer. Good luck!